1. Field of the Invention
The present invention relates to a detector for detecting an electromagnetic wave (THz-wave) in a THz-frequency band and more particularly to a bolometer-type THz-wave detector.
2. Description of the Related Art
Recently, an electromagnetic wave in a terahertz frequency band between light and an electric wave (that is, an electromagnetic wave with a frequency of 1012 Hz and a wavelength of approximately 30 μm to 1 mm, hereinafter referred to as THz-wave) has drawn attention as the electromagnetic wave directly reflecting information of a substance. A detector for detecting the THz wave (hereinafter referred to as a THz-wave detector) is generally in a structure comprising an antenna portion such as a dipole antenna or a Bow-tie antenna capturing the THz wave and an electric signal conversion portion for converting the THz wave captured by the antenna portion into an electric signal. As methods of converting the electromagnetic wave into the electric signal, Capacitive coupling type, Resistive coupling type and the like are known.
U.S. Pat. No. 6,329,655 discloses, for example, a Capacitive-coupling type THz-wave detector as shown in FIGS. 21A and 21B. This THz-wave detector is in a structure in which a glass layer 31 is formed on a substrate 30, four metal antennas 32 (Bow-tie antennas) are formed on the glass layer 31, and a detecting element 37 in which a heater film 33, an insulator 34, a thermal resistance layer 35, and an insulator 36 are laminated, is formed at the center part of the four metal antennas 32, with a predetermined gap (GAP 1 and GAP 2) from the glass layer 31 and the metal antennas 32.
U.S. Pat. No. 6,329,655 also discloses a Resistive-coupling type THz-wave detector as shown in FIGS. 22A and 22B. This THz-wave detector is in a structure in which the glass layer 31 is formed on the substrate 30, the four metal antennas 32 (Bow-tie antennas) are formed on the glass layer 31, and the heater film 33 connected to the four metal antennas 32 is formed at a distance of a predetermined gap (GAP 3) from the glass layer 31, and the detecting element 37, in which the insulator 34, the thermal resistance layer 35, and the insulator 36 are laminated on the heater film 33, is formed. In the structure of the Resistive-coupling type THz-wave detector, a leg 38 with an impedance matched to 50 to 100Ω is needed in order to effectively transmit energy collected by the metal antennas 32 to the heater film 33, and heat conductance becomes large. Therefore, it is described that sensitivity of the Resistive-coupling type THz-wave detector is lower by one order of magnitude than that of the Capacitive-coupling type THz-wave detector.
In the case of detection of a THz wave by the Capacitive-coupling type THz-wave detector, efficient transmission of energy collected by the metal antennas 32 to the heater film 33 is required. For that purpose, a gap between the glass layer 31 and the detecting element 37 (GAP 1) and a gap between the metal antenna 32 and the detecting element 37 (GAP 2) should be controlled accurately. The above U.S. Pat. No. 6,329,655 describes that a scope of 0.1 to 1 μm is preferable as the value of GAP 2. However, if the detecting element 37 is to be suspended from the glass layer 31 by the leg 38 using the MEMS (Micro-Electro-Mechanical Systems) technology, it is difficult to set the gap within the range of 0.1 to 1 μm, and there is a problem that yield is lowered.
In the case of detection of the THz wave by the Resistive-coupling type THz-wave detector, efficient transmission of the energy collected by the metal antennas 32 to the heater film 33 is also required. For that purpose, the gap between the glass layer 31 and the heater film 33 (GAP 3) should be controlled accurately. The above U.S. Pat. No. 6,329,655 describes that a scope of 0.2 to 1 μm is preferable as the value of GAP 3. However, if the heater film 33 is to be suspended from the glass layer 31 using the MEMS technology, it is difficult to set the gap within the range of 0.2 to 1 μm, and there is a problem that yield is lowered.
It is also known that an effective aperture that can capture the electromagnetic wave by the antenna, becomes an aperture merely of a circle with a radius of merely a half wavelength at the most. It is necessary to increase the size of the metal antenna 32 to efficiently capture the THz wave, but if the THz-wave detector in the above structure is made into a two-dimensional array, the size of each detector is limited. Therefore, the size of the detecting element 37 inevitably becomes small. For example, with the THz wave with the wavelength of 1 mm, the size of the detecting element 37 is approximately several μm. It is extremely difficult to incorporate the detecting element 37 in such a small region of several μm, and there is a problem that the yield is further deteriorated.